Pascal
Bertsch
*a,
Alexandra
Thoma
a,
Jotam
Bergfreund
a,
Thomas
Geue
b and
Peter
Fischer
*a
aInstitute of Food Nutrition and Health, ETH Zurich, 8092 Zurich, Switzerland. E-mail: pascal.bertsch@hest.ethz.ch; peter.fischer@hest.ethz.ch; Tel: +41 44 632 85 36
bLaboratory of Neutron Scattering and Imaging, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland
First published on 9th July 2019
The formation of electrostatic protein–polysaccharide multilayers has attracted attention for the design of fluid interfaces with enhanced stability and functionality. However, current techniques are often limited to measuring final multilayer properties. We present an interfacial shear rheology setup with simultaneous subphase exchange, allowing the transient measurement of biopolymer multilayers by their viscoelasticity. The successive and simultaneous adsorption of β-lactoglobulin (β-lg) and low-methoxyl pectin were investigated at the n-dodecane/water interface at pH 4. The successive injection of pectin increased the viscoelasticity of an adsorbed β-lg layer by electrostatic complexation. On the other hand, simultaneous adsorption impeded adsorption kinetics and interfacial layer strength due to complexation in the bulk phase prior to adsorption. Neutron reflectometry at the air–water interface confirmed the formation of an initial β-lg layer and electrostatic complexation of a secondary pectin layer, which desorbed upon pH-induced charge inversion. The layer formed by simultaneous adsorption mainly consisted of β-lg. We conclude that protein–polysaccharide complexes show limited surface activity and result in a lower effective protein concentration available for adsorption.
Here, we use β-lactoglobulin (β-lg), a globular protein fractionated from whey.8 The adsorption of globular proteins at fluid interfaces comprises three consecutive subprocesses: (i) diffusion limited transport to the interface,9 (ii) conformational changes and adsorption limited by the proteins' thermodynamic stability,1,10 and (iii) interfacial rearrangement.11 The functional structure of β-lg regarding interactions with lipids is a hydrophobic β-strand barrel called calyx.12 Because of the affinity of the calyx towards hydrophobic substances, the adsorption of β-lg strongly depends on the hydrophobicity of the employed subphase, as shown in molecular dynamics simulations13 and experimentally.14 β-lg has a pH-dependent charge with an isoelectric point ≈5, where it forms the most compact interfacial layers due to minimum intermolecular repulsion.15
Most polysaccharides are not surface active and do not adsorb at fluid interfaces. An established approach is the adsorption of a primary protein layer followed by electrostatic complexation of a secondary polysaccharide layer, as employed here. Alternatively, proteins and polysaccharides may be covalently conjugated.16–18 In case of electrostatic complexation, the formation of a stable protein layer prior to polysaccharide addition is crucial, as complexation in the bulk impedes adsorption kinetics and results in weaker interfacial layers compared to pure protein layers.5,19 On the other hand, the successive adsorption of a polysaccharide at an equilibrated protein layer increases interfacial layer strength, as demonstrated by Ganzevles et al.5 by an increase in interfacial shear moduli. However, interfacial rheology measurements are usually restricted to measuring final layer properties, stressing the need for transient interfacial rheology measurements during multilayer formation.
Here, we employ an interfacial shear rheology setup with simultaneous subphase exchange. This allowed the formation of pure β-lg layers followed by injection of low-methoxyl pectin while measuring interfacial shear rheology. The interfacial viscoelasticity during simultaneous and successive injection of β-lg and low-methoxyl pectin was investigated and complemented with dynamic interface tension, interfacial dilatation, and neutron reflectivity measurements.
Experiments were performed at pH 4 in 1/10 McIlvaine buffer mixed from 10 mM citric acid and 20 mM disodium hydrogen phosphate. The protein or pectin was added to the buffer an the pH adjusted using 1 M HCl and NaOH, respectively. The zeta-potential of β-lg and pectin as a function of pH was determined by a Zetasizer nano ZS (Malvern, U.K.).
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Fig. 1 Zeta-potential as a function of pH for 1 wt% β-lg and 0.1 wt% pectin in 1/10 McIlvaine buffer. Lines are to guide the eye. |
We employed an interfacial shear rheology setup with simultaneous subphase exchange,21 allowing the successive injection of β-lg and pectin while measuring interfacial shear rheology. This method was recently demonstrated to allow transient measurements of dynamic interfacial moduli Gi′ and Gi′′ during β-lg adsorption.14Fig. 2B depicts the interfacial layer formation of pure β-lg with successive pectin addition compared to their simultaneous injection. The injection of 50 mL 0.1 wt% β-lg solution (0.5 mL min−1) was initiated after 5 min and a measurable viscoelastic network was formed after 20 min. Note that a peristaltic pump was attached to provide constant mixing, resulting in a significantly shorter lag-phase compared to our previous investigation of diffusion limited β-lg adsorption.14 After 24 hours, 20 mL 0.1 wt% pectin were injected to the equilibrated β-lg layer, resulting in a twofold increase in interfacial moduli. Thus, the electrostatic complexation of pectin at an adsorbed β-lg layer increases its interfacial viscoelasticity. This is in contrast to simultaneous injection of β-lg and pectin. The electrostatic complexation in the bulk prior to adsorption results in a prolonged lag-phase, slower adsorption, and lower Gi′ and Gi′′ compared to the pure β-lg layer. The adverse effect of protein–polysaccharide complexation on adsorption kinetics is in agreement with dynamic interface tension measurements and was reported before.5,19 It is attributed to the slower diffusion, lower affinity for adsorption, or even precipitation of the β-lg–pectin complexes. In our case with constant mixing diffusion probably played a minor role, underlining the importance of the latter two. The adsorption limiting effect of polysaccharides is enhanced at increasing polysaccharide charge density,28 and may be counteracted by increasing the β-lg/pectin ratio.5
Fig. 2C depicts strain sweep experiments of the formed biopolymer layers. The mixed β-lg–pectin layer formed after simultaneous adsorption showed the lowest critical strain with yield at 1%. Pure β-lg and the successively adsorbed β-lg–pectin layer showed similar critical strains at ≈2.7%. Thus, the additional adsorption of pectin increases the dynamic interfacial moduli without lowering the critical strain, which is often observed for layers at increasing interfacial moduli.7 An overview of equilibrated Gi′ and critical strain of the different biopolymer layers is provided in Fig. 2D. Compared to pure β-lg, Gi′ was increased twofold by the successive adsorption of pectin. Comparable results were obtained for the successive adsorption of other proteins and polysaccharides under associative conditions.16,29,30 The interfacial elasticity of successively adsorbed β-lg and pectin was five times higher compared to simultaneous adsorption, in good agreement with findings of Ganzevles et al.5 who found up to sixfold stronger interfacial layers. We used a β-lg/pectin mixing ratio of 1:
1. Ganzevles et al.5 found that at β-lg/pectin ratios higher than five the adverse effects of pectin on adsorption kinetics and interfacial viscoelasticity diminish. For successive adsorption on the other hand, the mixing ratio does not affect layer properties. In contrast to our results, the complexation of longer protein fibrils and pectin in the bulk may even increase the interfacial viscoelasticity compared to pure protein fibril layers.31 The presence of polysaccharides may also increase the strength of protein layers at neutral pH when both are charged negatively. This is attributed to a limited thermodynamic compatibility which favors protein aggregation at the interface.32–34 On the other hand, interfacial protein layers are weakened if polysaccharides are surface active themselves33 or covalently conjugated to adsorbed proteins.35 Interfacial rheological measurements with simultaneous subphase exchange presented here facilitate the determination of such complex synergistic and antagonistic biopolymer interactions, as it allows the transient assessment of interfacial moduli and is not limited to measuring final layer properties.
The successive adsorption of a protein and a polysaccharide is exploited in so-called layer-by-layer emulsions. Besides increased interfacial moduli, the secondary polysaccharide layer increases steric repulsion which may enhance emulsion stability.36 However, adsorbed polysaccharides may also destabilize emulsions by bridging flocculation or depletion.4,17,34 Besides emulsion stability, layer-by-layer adsorption can be employed to create functional emulsions for specific needs. The adsorption of a secondary polysaccharide layer increases heat resistance,2 allows the modulation of emulsion zeta-potential,3,4 and can prevent the adsorption of enzymes for gastric stable emulsions.6,7
Fig. 3A depicts dilatational amplitude sweeps of pure β-lg layers compared to multilayers formed by successive and simultaneous pectin injection. All layer showed mostly linear behavior with only a slight decrease in dilatational storage modulus E′ at increasing deformation. The loss modulus E′′ was lower at all deformation, but was not depicted in Fig. 3A for clarity. The successive electrostatic complexation of pectin resulted in an increase in E′ compared to the pure β-lg layer. In contrast, the complexation in the bulk prior to adsorption resulted in lower E′. Thus, the same effects of simultaneous and successive pectin complexation are observed in dilatational rheology at the A/W interface as in shear rheology at the O/W interface discussed above. The interfacial elasticity can not be further enhanced by increasing the concentration of successively added pectin, as demonstrated in Fig. 3B. It is known from interfacial shear rheology that the β-lg/pectin ratio does not alter the interfacial properties if pectin is added successively.5
The adsorption of biopolymer layers at the A/W interface further facilitates to probe their structure using neutron reflectivity. Fig. 4 shows the fitted neutron reflectivity, reflectivity relative to the reference RRel, and scattering length density (SLD) profiles of the respective layers at the air/D2O interface.
The biopolymer layers depicted in Fig. 4A were formed in three consecutive steps. (i) A pure β-lg layer was formed at 0.001 wt%, which was best described by a monolayer with a thickness of 14 Å. This corresponds to a loosely packed monolayer, considering the diameter of native β-lg in solution of approximately 40 Å. However, protein unfolding and spreading at the interface may result in a more compact β-lg structure at the interface.10,14 Previous studies reported β-lg monolayers in the range of 20–40 Å.7,38,39 The thinner layer formed here can be attributed to a tenfold lower β-lg concentration, which was chosen to avoid excess β-lg in the bulk and prevent complexation with pectin. (ii) 0.002 wt% pectin was successively injected to the adsorbed β-lg layer. The obtained interfacial layer was best described by a two-layer model consisting of the primary β-lg layer followed by the consecutively added pectin. The β-lg layer was compressed to 11 Å and had a higher surface roughness compared to the initial layer, as denoted by a shift in the SLD profile towards the air phase. Hence, the initial β-lg layer is compressed and pushed towards the hydrophobic subphase by the consecutive adsorption of pectin. The adsorbed pectin layer was 5 Å in thickness. The thickness of the pectin backbone is 3 Å,40 indicating a compact pectin layer with limited extension into the water phase. The dense pectin layer is probably the reason why higher pectin concentrations did not increase interfacial dilatational moduli (Fig. 3B). The structure of successive β-lg–pectin layers may be different in emulsions, as pectins can adsorb at more than one droplet, particularly at high molecular weight. This may induce bridging flocculation, but could also provide additional bulk stabilization. In a third step (iii), the pD was increased to 7 to investigate if the electrostatic complexation of pectin is reversible upon charge inversion of β-lg. The obtained layer was indeed best described by a β-lg monolayer with a thickness of 14 Å, same as the initial β-lg layer. Thus, pectin desorbed due to electrostatic repulsion. The remaining β-lg layer was rougher compared to the initial layer. It can not be conclusively stated if this derives from irreversible structural changes due to pectin adsorption or traces of remaining pectin. Adsorbed protein layers are known to undergo structural changes at varying pH15 or in presence of like-charged polysaccharides.32–34 Thus, interfacial electrostatic β-lg–pectin complexes are reversible, as previously demonstrated in bulk experiments.27 This pH-triggered adsorption and desorption of pectin at protein-stabilized interfaces could allow the production of pH-sensitive colloidal materials. For example, it has been demonstrated that secondary polysaccharide layers can prevent the digestion of protein-stabilized emulsions under gastric conditions.6,7 The pH-triggered charge inversion in the duodenum could induce the desorption of this protective layer for controlled delivery applications.
In Fig. 4B the neutron reflectivity of a β-lg layer formed at 0.001 wt% is compared to a layer of simultaneously adsorbed β-lg and pectin at each 0.005 wt%. The β-lg layer was a 14 Å monolayer as discussed in detail above. Interestingly, the layer formed during simultaneous adsorption of 0.005 wt% β-lg and pectin was also best described by a monolayer 14 Å in thickness, and fit goodness increased when the SLD was changed towards the one of β-lg. This suggests that the interfacial layer formed during simultaneous adsorption consists mostly of β-lg. The layer roughness was slightly increased similar to the layer after charge inversion at pD 7 discussed above, potentially due to the presence of a minor pectin fraction. Overall, the layer was similar to the one formed with pure β-lg at fivefold lower concentrations. We conclude that β-lg–pectin complexes formed in the bulk show a low tendency towards adsorption, and the complexation reduces the effective β-lg concentration available for adsorption. Thus, the impeded adsorption kinetics and interfacial viscoelasticity discussed above could derive from a decreased effective protein concentration. This theory is supported by previous findings who demonstrated that the adverse effects of pectin on adsorption and interfacial layer strength can be overcome by increasing the β-lg concentration.5 However, interfacial layers formed at higher β-lg/pectin ratios also showed a different structure.39 A dense β-lg layer followed by a loosely packed secondary layer of β-lg–pectin complexes was observed at tenfold higher β-lg concentrations and β-lg/pectin ratios of two to six. This is in contrast to the β-lg dominated monolayer observed here for a mixing ratio of one.
Structure analysis by neutron reflectometry confirmed the formation of an initial β-lg layer followed by electrostatic complexation of a secondary pectin layer. The complexation of pectin was reversible upon pH-induced charge inversion, posing interesting applications for pH-sensitive colloidal materials. For the simultaneous adsorption of β-lg and pectin we observed a thin monolayer which consists mostly of β-lg. We concluded that electrostatic complexes decrease the effective protein concentration available for adsorption and show limited surface activity themselves.
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